Abbasid rule began in 750 and ended in 1258. They moved the Capital from Damascus to Baghdad and it grew to a population of about 2 million under their rule. Most of the caliphs were enlightened, valued scholarship and spend heavily on the institutions of learning and education. It is estimated that their expenditure on education in relation to their revenue was much higher than the governments of the most advanced countries spend on education today. This was a major factor that led to the golden age(8th to 13th century). Below is a link to a discussion on Abbasid Caliphate on a BBC rodio 4 on program called “In Our Times”.
Golden age of Islam, Abbasid Caliphs, a link to BBC radio 4. In our times
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The ABCs of Islam and Sufism 